1. Field
The disclosed concept pertains generally to circuit interrupters and, more particularly, to arc fault circuit interrupters. The disclosed concept also pertains to arc fault detectors for arc fault circuit interrupters and, more particularly, to operational amplifier circuits for such arc fault detectors.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
An arc fault circuit interrupter (AFCI) is a device intended to mitigate the effects of arc faults by functioning to deenergize an electrical circuit when an arc fault is detected. Non-limiting examples of AFCIs include: (1) arc fault circuit breakers; (2) branch/feeder arc fault circuit interrupters, which are intended to be installed at the origin of a branch circuit or feeder, such as a panelboard, and which may provide protection from series arc faults, ground faults and line-to-neutral faults up to the outlet; (3) outlet circuit arc fault circuit interrupters, which are intended to be installed at a branch circuit outlet, such as an outlet box, in order to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing, and which may provide protection from series arc faults, line-to-ground faults and line-to-neutral faults; (4) cord arc fault circuit interrupters, which are intended to be connected to a receptacle outlet, in order to provide protection to an integral or separate power supply cord; (5) combination arc fault circuit interrupters, which function as either a branch/feeder or an outlet circuit AFCI; and (6) portable arc fault circuit interrupters, which are intended to be connected to a receptacle outlet and provided with one or more outlets.
United States Patent Application Publication No. 2008/0204949 discloses an AFCI including a number of circuits each having a two-stage band pass filter, a peak detector and an envelope detector in combination with a current sensor, such as a current transformer (CT). For example, if two CTs are employed, then the CTs not only measure two high frequency signals from the sensed current, but also measure the power frequency signal from the sensed current. The CTs sense di/dt and their outputs are integrated over a finite time period to obtain a current signal. Two CTs can be employed to optimize outputs in corresponding desired frequency ranges, in order that significant amplification of the CTs' outputs is avoided. The CT outputs are input by corresponding, different two-stage band pass filters. The outputs of the filters are input by two peak detectors, and by two envelope detectors. The outputs of the peak detectors are input by input channels of an analog-to-digital converter (ADC) for a processor. The outputs of the envelope detectors are input by negative inputs of two comparators, which use references at their positive inputs. The outputs of the comparators are input by an OR gate, and the OR gate output is input by a counter of the processor. Plural high frequency, relatively narrow bands of the corresponding band pass filters are completely separated and do not overlap. The peak detector outputs of the high frequency signals above predefined thresholds are either ‘OR’ed or ‘AND’ed to determine if there is an arc fault.
Referring to FIG. 1, an AFCI 2 includes two high frequency operational amplifiers 4,6 and two Schottky diodes 8,10 to detect high frequency (e.g., about 1 MHz) arcing signals. For example, an LM6646 dual operational amplifier has a 55 MHz gain-bandwidth (GBW), 22 v/μs slew rate, and a maximum power consumption of 1400 μA per operational amplifier (or 2800 μA total). An arc fault detection (AFD) circuit 12 detects the peak high frequency signal and counts the number of bursts of high frequency for each half cycle. However, the AFD circuit 12 completely loses signal information at relatively low signal levels due to the forward voltage of the Schottky diodes 8,10. Hence, information as to the minimum high frequency signal and the duration of the high frequency signal can be lost (e.g., high frequency signals below about 20% of full scale).
There is room for improvement in arc fault circuit interrupters.
There is also room for improvement in operational amplifier circuits for arc fault circuit interrupters.